An ATM network (Asynchronous Transfer Mode) is a network in which the transmission of informations is based on the asynchronous time multiplexing of information packets, called cells, of fixed length. It is possible to transport, on such a network, any service, such as an audio, a video or a data transmission service, independently of its intrinsic characteristics such as, for example, the binary thruput, its quality characteristics or its sporadic nature. It is for these reasons that this technique of cell transfer by asynchronous time multiplexing has been retained as transfer mode by the wide band numerical networks with integration of services, called BISDN networks.
Nevertheless, in order to guarantee the service quality and to supply the passing band required by the applications, it has proved necessary to provide for various controls. This proves especially true for cases of traffic with highly sporadic thruput, such as those caused by the interconnection of local networks.
These controls relate, for the one part, to the network parameters controls, or to the controls of use parameters, that relate to the aggregate of the actions performed by the network to oversee and control traffic on an ATM connection. The invention does not relate to this type of controls. It does relate, however, to the control of a connection acceptance, a control, that aims at accepting or refusing a connection, when its establishment is requested, on the basis of whether or not the quantity of available resources permits to transport the new connection through the entire network, with the required quality of service.
Thus, at the time a connection is established, there is passed, between the user and the network, a traffic contract that comprises on the one part a traffic descriptor of the connection under consideration and, on the other part, the required service quality level (QoS). These informations are supplied by the user, at the time the connection is established, by means of signalling for example.
The traffic descriptor of the connection that is being established may comprise the following four traffic parameters: the maximal cell thruput of peak rate (PCR, Peak Cell Rate), the variation in the tolerance of the cell delay, or gigue tolerance (to.sub.PCR), and possibly the projected cell rate (SCR, Sustainable Cell Rate) and its associated tolerance, to.sub.SCR.
By subscribing to the contract, the user assumes the obligation that the traffic he sends will comply with the traffic defined by the parameters of the contract, while the network ensures a respect for thruput rate and for gigue tolerance as soon as this compliance is achieved. The obligation for the traffics to comply with their contract thus protects the network against the arrival of uncontrolled bursts of volume, and makes it possible to reserve, within the network, the resources needed to guarantee the required quality of service.
From the above-mentioned traffic parameters, four traffic contract definitions have been proposed, that are respectively called the deterministic bit rate mode, DBR; the statistical bit rate mode, SBR; the ATM block transfer mode, ABT; and the Available Bit Rate Mode, ABR. A review of these four definitions is found in the article of J. Mignault, A. Gravey and C. Rosenberg titled "A Survey of straightforward Multiplexing Models for ATM Networks: in ATM Expert RACE Symposium, 1995. " They also constitute the object of the recommendation I.371 of the ITU-T.
In the DBR mode, the source or user simply states, at the time of the connection, the maximal cell rate he intends to respect during the entire duration of the communication. The reservation of the network resources then takes place on the basis of this maximal rate, while the quality guarantees "QoS" are given in terms of delay and of rate of cell loss. This mode essentially is meant for connections with constant bit rate (CBR).
For connections that take on services with variable bit rate, VBR, the source or user can state, in the so-called SBR mode, a sustainable cell rate in addition to the peak rate, PCR.
From the traffic parameters negotiated at the time of the contract it is possible to estimate a passing band equivalent to the traffic of the connection involved, as a function of the link rate, of the available resources in the network, and of the required quality of service. The reservation of the resources thus is made in the network on the basis of this estimated passing band.
The ABT mode aims to protect the quality of service at the level of the rasters rather than at the level of the cells. In this mode, the source negotiates a maximal peak rate PCR.sub.max, for the duration of the call, and uses a negotiation in the band. The reservation of resources then is made on the basis of the lowest one, between the peak-rate PCR and the maximal peak-rate, PCR.sub.max values.
As for the ABR mode, it is meant for the sources of data.
The compliance of a cell with the traffic contract established by the user is defined by means of an algorithm such as the "leaky bucket" algorithm LB, or of its equivalent, the Virtual Scheduling algorithm (VS).
In the DBR mode, the traffic of a connection is in compliance wtih respect to these algorithms, as long as the length of a virtual waiting file, fed by the traffic of the connection under consideration and emptied at the negotiated peak rate PCR, does not exceed the maximal length L.sub.max defined with respect to the stated gigue tolerance to.sub.PCR by means of the following equation: EQU L.sub.max =PCR.times.to.sub.PCR.
In the SBR mode, the algorithmic definition of the sustainable rate SCR is similar to that of the peak-rate PAR, except that it is associated with a higher gag tolerance to.sub.SCR.
These traffic contracts generally are defined for the entire duration of the communication, this making difficult the optimal use of the network resources, especially in the case of sporadic traffics that are poorly defined in terms of the passing band over the entire duration of a communication. The traffic parameters, in the end, are therefore hard to define over the entire duration of the communication. There results from this that the reservations of resources for a peak-rate, that are valid for the entire duration of the communication, unavoidably cause a waste of resources, computed in terms of passing band.